1. Field of the Invention
The invention relates to a hydraulic control apparatus for a vehicular automatic transmission in which hydraulic frictional engagement devices are engaged by controlling solenoid valves that output hydraulic pressure directly to the hydraulic frictional engagement devices. More particularly, the invention relates to a technology in which when there is a failure that may cause tie-up in an automatic transmission, a specific shift speed is achieved to prevent tie-up.
2. Description of the Related Art
Japanese Patent Application Publication No. JP-A-2000-65203 describes an example of a hydraulic control apparatus for a vehicular automatic transmission. In the automatic transmission, a plurality of shift speeds having different speed ratios are achieved by selectively engaging and disengaging a plurality of hydraulic frictional engagement devices. The hydraulic control apparatus includes a plurality of solenoid valves that correspond to the respective hydraulic frictional engagement devices, and output hydraulic pressure directly to the respective hydraulic frictional engagement devices. The hydraulic control apparatus achieves the plurality of shift speeds by controlling the plurality of solenoid valves.
In this hydraulic control apparatus, if a given solenoid valve remains in the position to output the hydraulic pressure (i.e., a given solenoid valve continues to output the hydraulic pressure) due to a failure, the hydraulic frictional engagement device corresponding to the failed solenoid valve is constantly maintained in the engaged state by the hydraulic pressure output from the failed solenoid valve. In this case, at least one hydraulic frictional engagement device may be engaged in addition to predetermined frictional engagement devices that are engaged to achieve a current shift speed, and the gear mechanisms of the automatic transmission may be locked. That is, so-called tie-up may occur in the automatic transmission. To prevent this tie-up, the shift speed is changed to another shift speed that is achieved on the condition that the hydraulic frictional engagement device corresponding to the failed solenoid valve is engaged.
That is, in the conventional apparatus, tie-up is prevented by changing the shift speed to another shift speed that is achieved on the condition that the hydraulic frictional engagement device corresponding to the failed solenoid valve is engaged by the hydraulic pressure output from the failed solenoid valve. However, if the shift speed is changed to another shift speed that is achieved on the condition that the hydraulic frictional engagement device corresponding to the failed solenoid valve is engaged, a shift that may impair driveability may be executed, for example, a drastic downshift from the eighth speed to the fifth speed may be executed. This may make the driver feel uncomfortable.